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Overdiagnosis more likely than early detection of threatening cancer

Overdiagnosis more likely than early detection of
threatening cancer with mammography screening
Publish date: October 13, 2016

Author(s): Jessica Craig

Following the introduction of
mammography, women were considerably more likely to have
tumors that were overdiagnosed than to have earlier
detection of a tumor that was destined to become large,
according to an analysis of trends from the Surveillance,
Epidemiology, and End Results (SEER) database.

To assess the effectiveness
of screening mammography in the real-world clinical and
community setting, H. Gilbert Welch, MD, of the Dartmouth
Institute for Health Policy and Clinical Practice and his
associates examined trends in breast tumor size between the
years of 1975 and 2012, a span of time that can be broken
down into two distinct periods: a baseline period that
predated the widespread use of screening mammography
(1975-1979) and a period encompassing the most recent years
for which 10 years of follow-up data were available (2000
through 2002).

“Although the biologic
characteristics of a tumor are now recognized to be more
relevant to breast cancer prognosis than the size of the
tumor, tumor size is more relevant to the assessment of the
proximate effect of screening,” the researchers explained (N
Engl J Med. 2016 Oct 12;375[15]:1438-47).

Retrospective analysis of
the SEER database revealed a shift in the size distribution
of breast tumors: Large tumors, defined as invasive tumors
measuring two centimeters or more, predominated in the
period before widespread screening mammography, and small
tumors, defined as in situ carcinomas or invasive tumors
measuring less than two centimeters, predominated after.

This shift, the researchers
noted, can, in part, be attributed to the use of screening
mammography.

Overall, from 1975 to 2012,
the proportion of breast tumors that were small increased
from 36% to 68%. In that same time period, the proportion of
large tumors decreased from 64% to 32%.

“This shift in size
distribution was less the result of a substantial decrease
in the incidence of large tumors and more the result of
substantial increases in the detection of small tumors,” Dr.
Welch and his associates wrote.

Put another way, the
incidence of small tumors increased by 162 cases of cancer
per 100,000 women, from 82 to 244 cases, while the incidence
of large tumors decreased by 30 cases of cancer per 100,000
women, from 145 to 115 cases. “Assuming that the underlying
burden of clinically meaningful breast cancer was unchanged,
these data suggest that 30 cases of cancer per 100,000 women
were destined to become large but were detected earlier, and
the remaining 132 cases of cancer per 100,000 women were
overdiagnosed,” the researchers wrote.

“The magnitude of the
imbalance indicates that women were considerably more likely
to have tumors that were overdiagnosed than to have earlier
detection of a tumor that was destined to become large,” the
researchers continued. “Our analysis of size-specific
incidence highlights the fact that the introduction of
screening mammography has produced a mixture of effects,”
Dr. Welch and his associates added.